ABSTRACT
A tower top amplifier (TTA) can be used to improve
base station receiver sensitivity by compensating for heliax cable losses.
Should a failure occur, such devices are difficult to access and repair due to
their mounting location at the tops of tall structures.
Some TTA designs include an automatic bypass switch
in case of amplifier failure. Although system performance is severely impaired
by bypassing the amplifier, this prevents total loss of receiver operation.
This paper describes a design enhancement that
compensates for a TTA failure by switching in a redundant preamplifier located
in the equipment cabinet at the base of the tower. With this improvement only a
minor degra-dation occurs to receiver performance.
PROBLEM
Tower top amplifiers are sometimes used to maximize
receiver sensitivity by placing a low-noise amplifier (LNA) within a few feet
of the antenna. This improves receiver sensitivity (at great expense) by
preventing heliax losses from playing a significant role in establishing the
system noise figure.
In practice, it is difficult and expensive to
access TTAs at the tops of towers. It can be hours or days before a tower crew
can be assembled to make repairs.
Some TTA designs may include automatic switching to
activate a back-up amplifier or (if that fails) bypass all failed amplifiers.
System performance is severely impaired when a failed amplifier is bypassed
because there is inadequate gain in the overall system lineup. Bypassing is
still preferable to doing nothing, as a failed amplifier will cause a severe
degradation to the system noise figure by introducing several dBs of additional
loss. Bypassing a failed amplifier prevents total loss of receiver operation.
It is very desirable to compensate for the lack of gain when a failed amplifier
i...
